For building large constructions as e.g. houses, walls, tower elements, and similar building structures, typically a foundation supported by a number of foundation piles is used, where the piles are placed in the ground for supporting the foundation and for absorbing the compressing and tensile forces caused by the constructions due to their dead weight and wind load.
For absorbing the compressive forces, typically smooth foundation piles are used that are driven down into the earth until they hit a firm substratum. This implies that in some places, many meters of foundation pile are to be used before the bottom reaches a firm bed. Therefore, this method may be a very expensive way of founding a building construction on.
By building in areas where earth surveys show that a firm bed is far down, another type of foundation pile is used, where the foundation pile is provided at its lower end with a footing element having a diameter larger than the diameter of the foundation pile itself. This means that the foundation pile is provided a large area over which the pressure is distributed, and that the footing element makes it more difficult for the pressure caused by the dead weight of the building structures to press the foundation pile farther down into the ground.
Since it is not possible to drive down a foundation pile with enlarged footing element or to press such a foundation pile down through a pre-drilled hole, different solutions have been found as to how the foundation pile is disposed in the desired position before the enlarged reinforcement unit is produced.
Such a foundation pile is known from U.S. Pat. No. 3,832,859 where a foundation pile has a reinforcement unit designed so that by driving down it may spread a number of legs out into the earth layer, after which the pile foot may be cast for formation of a footing element with a dimension greater than the diameter of the foundation pile.
A drawback by using either smooth foundation poles or foundation piles with enlarged footing elements compared with the outer periphery of the foundation pile as described in U.S. Pat. No. 3,832,859 is that these foundation piles are not suited for absorbing tensile forces.
These tensile forces arise when a building section or a tower element is for example subjected to wind forces, whereby the foundation piles in the lee side will be exposed to compressive forces while the foundation piles to windward will be exposed to tensile forces.
If the foundation piles can absorb tensile forces, it is an advantage that the foundation piles longitudinally have one or more beads and/or a footing element, which have greater diameter than the foundation pile itself, at one or more points. The increased diameter of the beads/footing element compared with the foundation pile implies that the foundation pile may absorb larger tensile forces than a smooth foundation pile, as the beads/footing element are acted on by the mass of earth that is distributed in an upwards directed conical shape above the beads/footing element.
By using smooth foundation piles, these will only absorb tensile forces provided by the suction of the earth. This suction appears when a foundation pile, due to the resisting force between the surrounding earth layer and the surface of the foundation pile, is pulled out from it position, whereby subpressure occurs in the hole from which the foundation pile is removed.
There are different ways in which the beads/footing element may be provided in connection with a foundation pile placed in the ground. One possibility is that the foundation pile has a longitudinally through-going duct through which there is pressed a curable filler out into the earth, either at the lower part of the foundation pile or via transverse filling ducts along the length of the foundation pile.
In order that these beads footing element can withstand the increased tensile strength, it is typically reinforced with a reinforcement unit. In U.S. Pat. No. 3,832,859 is described how a reinforcement unit is designed so that by pressing down the unit spreads a number of legs out into the earth layer which subsequently are cast for formation of a footing element with a larger diameter than the diameter of the foundation pile.
A disadvantage of using such a reinforcement unit with the footing element of the foundation pile is that in this case the reinforcement will be disposed at the top of the reinforcing unit, which implies that the reinforcement is poorly distributed in the footing element and thereby is only attained a slight increase in the tensile strength. In order to attain optimal strength of the footing element, the reinforcement members of the reinforcement unit are to be distributed over the largest possible area of the footing element.
By using a foundation pile according to U.S. Pat. No. 3,832,859, the foundation pile will have a certain strength in relation to the tensile forces, but particularly in clay ground it will be prominent due to its design, and the foundation pile may release itself from the cast footing element, and thereby the design of the reinforcement unit will provide that the legs spread in the earth layer by upwards pull in the foundation pile will not hold, and thereby the reinforcement unit will fold with the result that the same resistance against the tensile forces is achieved as by a smooth foundation pile.